Prior to this invention, the only way that the performance of a "read circuit" for a magnetic recording data channel could be tested was either with a sine wave oscillator, or else with output from the actual head flying over a ("written") disk. Both of these methods have disadvantages, as workers know.
1. If a sine wave oscillator is used to test a "read circuit", the circuit may respond differently to the "head signal" than it does to the sine wave. This is because a head signal consists of a series of positive-going and negative-going pulses which have many harmonic components. FIGS. 1A and 1B are (oscilloscope) photographs of head signals typical of such instances. The two photos show the differences in readback wave forms that are solely due to the radial head position over the disk.
FIGS. 2 and 3 show the spectral content of these signals (taken at the inner and outer radius, resp., on a "test disk"). The vertical spikes represent the "fundamental" and its associated harmonics. The large spike at the left (about 2.8 MHz) is the fundamental and the harmonics shown are the 2nd, 3rd, 4th, 5th, 6th and 7th, etc. As a comparison, FIG. 4 shows the spectral content of a 3.0 MHz sine wave signal. Because it does not contain the same harmonics as the head signal, the circuit under test is not required to operate successfully over the bandwidth that is needed to handle actual waveforms.
2. The disadvantages to using an actual head and disk are many. First, and most important, is availability; for new technologies, the heads and/or disks may not exist at the time that the "read channel" development should take place. Furthermore, the heads and disks that may be available during read channel development may not be appropriate for their intended use. When this happens, the read channel may have to be continually modified during evolution of the head or disk technology.